Question: I have a Water Furnace and currently use a pleated filter with a MERV 5 rating.  how high can I go before I start restricting the flow too much and damage the system?

A Merv 5 filter is really not very efficient.  If you have a properly sized duct system and you change your air filter regularly I see no reason not to use an air filter with as high as a Merv 12 rating.  as Merv ratings increase so does restriction to air flow, so you always have to be careful.  The reason I mention a properly sized duct system is because if the duct system is undersized it will restrict air flow dramatically and is a common problem in many homes.  A very efficient air filter and an undersized duct system are not a good combination.

There are a couple of signs that may indicate an undersized duct system:

-  The air conditioner coil ices up when the filter becomes dirty.  Most systems will do this if the filter becomes extremely dirty but you should not have this problem if the filter becomes only moderately dirty and you change it regularly.

- Whistling sounds coming from around the air handler access panels can be an indication of an undersized duct system.  This can also indicate poorly fitting access panels.

- If your furnace trips of on the high limit switch, this could indicate an undersized duct system.

There are other things that could cause the problems listed above but they are also signs of restricted air flow.

If you have your equipment serviced regularly ask your contractor if the duct is sized properly.  Opinions vary among contractors regarding filters with high Merv ratings.  Your other option is to try a higher Merv rating and monitor equipment operation closely.

Question: What is the MERV Rating of the BoAir 5-Stage electrostatic filter?

The BoAir 5-Stage electrostatic filter like all electrostatic air filters doesn’t have a MERV rating. MERV ratings are only used to rate media types of air filters such as pleated filters and Hepa filters. But if you compare the Arrestance rating of the BoAir 5-Stage electrostatic filter, which is over 95%, the media type filters with similar Arrestance ratings it is equivalent to a Merv 6 or a Merv 8 filter.

A furnace filter with a Merv 8 rating is suitable for use in you air conditioning or heating system if you don’t suffer from allergies but if you require more efficient filtration to manage your allergies I would recommend a Merv 10 to 12 pleated filter.

Question: With regard to pleated air conditioner filters do you think I can go with a MERV 6 filter or do you think I will be better served going with a MERV 8 filter?

A MERV 8 filter will perform better than a MERV 6 filter but the difference will be minimal. If you are not too concerned about dust a MERV 6 filter will do a good enough job. If you are looking for a big improvement in dust control you may want to try a MERV 11 or 12 pleated filter.

Pleated filters in the MERV 5 to 8 range will stop particles of 3 to 10 microns such as some mold spores, fibers, dust mites, some bacteria and fiberglass among other things.

MERV 9 to 12 filters stop particles of 1 to 3 microns such as smoke, bacteria, spray, fumes, coal dust and auto emissions and many other things.

Question: My central air filter is inside the air handler in the attic. Due to a recent back injury, i cannot get up to the attic. Would I be able to remove the filter in the air handler and install a filtered return register?

That should not be a problem if you have only one return grill. Of course if you have more than one return grill you will have to install a filter in each.

If the return grill opening is smaller than your current filter size you may find that you have to change the filter more often because it will become dirty faster. You will have the same amount of air moving through a smaller filter and the dust will be collecting on a smaller surface area.

You may have to get somebody to modify the duct behind the return air grill to hold the filter in place but that shouldn’t be difficult. Or you may be able to modify the return grill to hold the filter if you can’t find a new one.

The only real concern that I might have is the impact that a smaller filter will have on air flow if the return air grill is smaller than the current filter size that you are using. This is something that could cause the evaporator to ice up due to reduced air flow and can have a negative impact on air flow. Changing or cleaning the filter regularly will be very important in this case.

We have all heard the horrifying story about the family that just barely escaped with their lives, or even worse the family that died in their sleep, there lives ended by a silent killer. The silent killer is CO (Carbon Monoxide). If you don’t change or maintain your furnace filter as recommended by the manufacturer you could be putting your family in danger. Your furnace filter is designed to remove dust from air as it passes through your furnace. The longer your furnace filter is installed the dirtier it becomes. This increased loading of the filter with dust causes a resistance to air flow through the filter and reduces air flow.

Gas and oil furnaces, like all heating and air conditioning equipment, are designed to have a certain amount of air flow at all times to operate properly and at highest efficiency. This air flow is measured in CFM (Cubic Feet per Minute). These appliances have a heat exchanger which has the heat source, flame, inside and the air passes over the heat exchanger on the outside. As the air passes over the heat exchanger it picks up the heat from the flame inside.

To get maximum efficiency from these systems the walls of the heat exchanger are made as thin as possible but not so thin that they cannot withstand the heat that they are subjected to during normal operation. This brings us back to the fact that these appliances are designed with minimum air flow requirements.

A furnace filter that remains in the system so long that it begins to negatively impact air flow can be dangerous. As the amount of air flowing over the heat exchanger declines, the operating temperature of the heat exchanger increases. This is when heat exchanger can crack because they are operating above design conditions. The hotter the heat exchanger gets the more it expands when heating and contracts as it cools during the off cycle. This expanding and contracting can become excessive and cause the metal of the heat exchanger warp or crack. A cracked heat exchanger can cause combustion gases to mix with the air in your home.

The good news is that properly designed furnaces have high limit switches that prevent excessive heating of the heat exchanger. These limit switches monitor the temperature at or near the heat exchanger and if the temperature reaches a predetermined set point, the unit will be shut off either until manually reset or until the limit switch senses that the temperature is within safe limits and resets automatically.

A furnace can operate a long time on an auto reset limit switch without anybody even knowing. This means that the furnace is operating at the limits of its design capabilities. Today most limit switches are the manual reset type, designed this way so that the furnace will not operate until it is serviced and the problem corrected.

Don’t rely on these safety switches to protect you from disaster. These are mechanical devices which can and do fail to operate properly at times. So reduce your risk by maintaining your furnace filter properly. This means cleaning or changing your furnace filter regularly. It is always best to follow manufacturers guidelines.